The rich history of binding studies of monoclonal Ig produced by plasmacytomas (PCs) have found no universally common binding properties, but instead, groups of PCs with specific antigen-binding activities to haptens such as phosphorylcholine, dextrans, fructofuranans or dinitrophenyl. Subsequently, it was found that PCs with similar binding specificities not only expressed the same idiotype but rearranged the same VL and VH genes to express a characteristic monoclonal antibody. In a study of silicone-induced mouse plasmacytomas (SIPCs), we have found; i). antibodies secreted by SIPCs bind to different antigens in a manner similar to that observed for natural autoantibodies, ii). the expressed Ig heavy genes are restricted in V gene usage to the VH-J558 family and iii) secondary rearrangements occur at the light chain level with at least three SIPCs expressing both kappa and lambda light chain genes. These results suggest that SIPCs utilize a restricted population of B cells that may still be undergoing rearrangement, thereby bypassing the allelic exclusion normally associated with expression of antibody genes. Mechanistically, if the secondary rearrangement is not from the same allele, a possibility could exist in which two light chains (kappa:kappa or kappa:lambda) can be simultaneously expressed, thereby leading to an apparent violation of allelic exclusion. Dual expressing lymphocytes have already been reported, but through the use of cell sorting which may not distinguish between one and two cells. To overcome this technical problem, we have developed a technique of micro-manipulation of cultured cells to obtain data for Ig, Ly1 and Rag1/2 expression by RT-PCR at the single-cell level. Our results show conclusively from multiple dissections that expression of kappa, lambda and Ly1 is associated with a single SIPC cell. While these dual-expressing cells represent a minor subset of the tumor population, the continued presence of these cells in many PCs, including MOPC104E, which has been in culture for more than 30 years, suggests an important role for these cells in development of the B cell repertoire. Based on the fact that the kappa:lambda cells are Ly 1+, represent a small, but stable percentage of the tumor cell population, exhibit limited N-region sequences and are located in the peritoneum, we suggest they may represent a B-1a self-renewing B cell. - B-1 cells, Antibody,